In modern wireless communications systems, in particular cellular telecommunications systems, the mobile station (also referred to, for example, as a wireless terminal or as a cellular telephone) must perform a number of tasks that are commanded by the base station. One of these tasks is to receive and measure the power level of different radio channels, such as those originating in neighbor cells, and to report the results of the measurements back to the base station. When performing this task the MS may be required to tune its receiver from a current channel to a channel to be measured, make the measurement, and then tune back to the original channel in order to continue an ongoing communication or to listen for a page. As can be appreciated, all of these operations need to be performed in short amount of time, in particular as some amount of settling time is typically required at least when re-tuning to the original communication frequency channel. Some settling time may be required as well when tuning to the channel to be measured.
A multi-band receiver is one that is capable of operating with at least two communication frequency bands (e.g., 1900 MHz and 800 MHz). If a multi-band receiver uses a common base band circuit for all the frequency bands, it is even more crucial to quickly recover from the channel measurement task. This is particularly the case if, for example, burst-type GSM (Global System for Mobile Communications) channels are required to be measured while a WCDMA (Wideband Code Division Multiple Access) call is in progress.
FIG. 1 shows a conventional architecture for a direct conversion receiver 1 having a front end 1A and a base band section 1B. RF signals are received through an antenna 2, amplified in a low noise amplifier (LNA) stage 3 and downconverted to baseband in a mixer stage 4, using a local oscillator input. The base band section 1B includes a base band gain block 5, a channel selection filter 6, an automatic gain control (AGC) block 7 and an analog to digital converter (ADC) 8 where the received signal is converted into digital data for subsequent processing in the digital domain.
The largest time constants of the direct conversion receiver 1 can typically be found in the AGC block 7. As such, the limiting factor on the other channel measurement task is typically related to the magnitude of the time constants inherent in the operation of the AGC block 7.
Referring also to FIG. 2, the AGC block 7, and a DC compensation circuit more particularly, is shown in greater detail. The AGC block 7 includes an amplifier 7A (shown connected in an inverting configuration), input resistor R1, feedback resistor R2 and an input DC blocking capacitor C1. Before the signal can be received with the direct conversion receiver 1, the DC-voltage of the output must be set to a proper value by using the DC compensation circuit of the AGC block 7. However, this circuit normally contains large capacitors (e.g., C1), which have to be charged to a proper value before reception. The charging of the capacitors requires some finite amount of time and also consumes current.
A further problem in the conventional direct conversion receiver relates to the biasing of the base band section 1B. In that the lowest signal frequency in the base band is 0 Hz (i.e, DC), care must be taken to avoid the occurrence of the wrong biasing of the base band section 1B when the gain is changed. This requirement results in the use of DC-compensation circuitry.
More particularly, the AGC circuitry 7 may not have the same gain for DC that it has for higher frequencies. This is true because the biasing of the base band section 1B may be changed each time the gain is changed. Providing a different value for the gain for operation at 0 Hz (DC) has traditionally been achieved through the use of large AC-coupling capacitors. As was discussed above, these capacitors must be charged to the correct value before each reception burst (as in a Time Division Multiple Access (TDMA) system such as GSM). Normally the charging is performed in a situation where some undesired signal is also present in the system. Therefore it is possible that not only the DC-value is charged to the capacitor, but that some of the undesired signal appears as well. This occurrence can result in the generation of a large and undesirable offset in the output of the base band section 1B.
Referring to FIG. 3, which corresponds to the R1/C1 input network shown in FIG. 2, in accordance with conventional practice the coupling capacitor (C) is charged as a 1st order lowpass filter, resulting in the AC value of the incoming error signal being reduced. The voltage across the capacitor (C) is thus given by the expression: Vcapacitor=(1/(1+sRC))(Vin−Vref).
However, a problem can arise in the case where the previous burst is significantly more powerful (for example 20 dB) than a current burst to be received. The first order lowpass A1 filter of FIG. 3 does not decrease the signal sufficiently, as the lowpass corner frequency cannot be decreased without increasing the time constant of the circuit, and hence increasing the required DC-compensation time (which is limited by system timing constraints).
Further reference with regard to direct conversion receivers, and to techniques for reducing the offset voltage and to base band signal processing in general, can be made to commonly assigned International Patent Application WO 97/29551, Method and Circuit Arrangement for Reducing Offset Voltage of a Signal, by Risto Väisänen, and to International Patent Application WO 97/29552, Method and Circuit Arrangement for Processing a Received Signal, also by Risto Väisänen, the disclosures of which are incorporated by reference herein in their entireties.